The present invention relates to compositions of free machining steels characterized by low sulphur concentrations and being essentially free of lead. More particularly, the invention is directed to products in which sulphur, bismuth and various combinations of tin, copper, barium and tellurium are added to and distributed through the molten steel. The resulting composite is then cooled to provide a solid metal ingot, bloom or billet convertible to a product having free machining properties. Included in the present invention are methods and apparatus for producing the free-machining products.
Numerous formulations of free machining steels are described in the relevant literature. Such formulations invariably include low melting elements added to improve machining characteristics. Sulphur is the most commonly used additive incorporated in free machining steels. In many of the prior art formulations, the concentration of sulphur is 0.30% or greater. High concentrations of sulphur are undesirable in that they cause defects and excessive surface discontinuities in the cast structure and in the hot worked product due to hot shortness or tearing.
Lead is the secondmost commonly used additive incorporated in free machining steels. Lead has two serious disadvantages:
Lead is toxic. Extreme caution must be taken during steelmaking, and any other processing steps involving high temperatures. Such processes produce lead and/or lead oxide fume. Atmosphere control procedures must be incorporated in high temperature processing of lead bearing steels. PA1 Lead is not uniformly distributed throughout the conventional steel products; it is not significantly soluble in the steel and, due to its high density, settles during the teeming and solidification process, resulting in segregation or non-uniform distribution within the as cast and final product.
It is a principal aim of the present invention to reduce the above and other objectionable features due to high sulphur and to the presence of lead, and to realize at the same time product improvements to ensure optimum machining properties in the steel produced.